Why we’re Pro-Proheart

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Why we’re Pro-Proheart

Why we’re Pro-Proheart


There are many different brands of heartworm preventatives on the market these days. Of these many brands, the vast majority contain the active ingredient milbemycin oxime. Why is this important information for you, a dog owner? Because, if you are one of the millions of owners who relies on monthly tablets for preventing heartworm, your dog likely isn’t actually protected against heartworm right now.

What are heartworms and how do they cause disease?

Cardiopulmonary dirofilariosis is a mosquito-borne parasitic disease caused by Dirofilaria immitis (heartworm). It affects both dogs and cats, as well as other domesticated species and wildlife. Just two decades ago, up to 80% of dogs in warmer climes such as the coastal areas of Australia had heartworm burdens. Heartworm is a dangerous parasite, as it can’t even be detected in the blood until it has been there for six months; this time period is known as the prepatent period. This means that, without effective heartworm prevention is used, a heartworm burden may be brewing in your pet with no signs for up to five years. Heartworms cause disease by growing and living in the cardiopulmonary system of our pets. They cause damage to the blood vessels, lungs and the heart itself. In some cases, the migration of the heartworm can rupture blood vessels or release emboli (blood clots) which can result in rapid and painful death.

Do I need to protect my pets against heartworm?

Short answer; yes! It is an unfortunate reality however that at least 10% of Australian owners do not give any sort of heartworm prevention. It is a common misconception that dogs in cities, or those which live indoors, do not require heartworm prevention. Heartworm, as mentioned above, is transmitted by mosquitos. Just one bite from a mozzie is enough to inoculate your pet with the deadly disease; why risk it? Treatment for established heartworm infections is high-risk, of a long duration and incredibly expensive. A once yearly proheart injection, is very affordable and is the absolute gold standard in heartworm prevention worldwide.

Why monthly dosing is a risky option

Recent survey studies conducted in the United States have revealed that of those patients not receiving the annual Proheart heartworm injection, only 75% are on a monthly preventative. Of these owners giving monthly heartworm preventatives, many of these people frequently forgot to dose their pets monthly or running out of their monthly product delayed administration. Of course this is a very understandable situation, however, this statistic is concerning, as it highlights how easily dogs can miss doses of their monthly heartworm prevention.

Disturbingly, a study recently conducted right here in Australia of all reported heartworm cases revealed that 40% of these patients were supposedly covered by a monthly preventative spot-on or tablet.
As mentioned above, even if your pet appears bright and well without heartworm prevention, they may well have several parasites growing within their cardiopulmonary system without an effective antiparasitic to eliminate them.

However, the most concerning thing about monthly heartworm preventative lies not in simple human error, but the active ingredient itself.
How is it possible that 40% of dogs with heartworm were supposedly protected against the deadly parasite?
The answer is simple; the preventative in question must be ineffective.

Milbemycin oxime: not what it seems

A vast majority of the oral and spot-on formulations for heartworm prevention in Australia contain the active ingredient milbemycin oxime.
An intensive multi-centre efficacy study was conducted by Holstrom et al in 2014. What they found was greatly concerning. The milbemycin oxime (MBO) was not even detectable in the bloodstream of the subjects after 14 days. This means that these animals receiving just MBO may not have had sufficient amount of the drug in their system to protect them against heartworm!

Furthermore, there have been several recent studies in Australia, New Zealand and the United States revolving around milbemycin oxime and another antiparasitic, ivermectin. Both of these drugs are a part of a group of drugs known as macrolytic lactones. They are routinely used for successful endoparasite (gastrointestinal worm) prevention and treatment in both domestic animals and livestock. What were are concerned with, however, is their use as heartworm treatments.

A study conducted by Snyder et al in North Carolina in the United States is one of these recent studies. The group conducted blind trials on groups of fourteen dogs to assess the efficacy of milbemycin oxime and ivermectin against a control group.
The study was conducted by inoculating all of the dogs with live heartworms, keeping one group untreated as the control and giving the other dogs either milbemycin oxime or ivermectin thirty days after the inoculation.
After a four-month period, necropsies were performed to assess how many heartworms persisted in both the control group (wherein the dogs would rely on their natural defences alone to deal with the heartworms) and the two treatment groups.
Not surprisingly, the control group dogs all had considerable heartworm burdens at the four-month mark.

Unfortunately, of the two groups of dogs which received either milbemycin or ivermectin, TWO dogs actually had heartworm! This means that, in theory, nearly 10% of dogs receiving either of these products marketed as a heartworm preventative may have heartworms. This study concluded that the claims of 100% protection put forward by any product with these active ingredients were inaccurate and unsupported.

But this is just one study, right? Surely these results are inaccurate.

A similar study was conducted by Blagburn et all in 2016. In their study, the team divided forty dogs into eight groups of five. One group served as control animals (group 1), one group was given a tablet with milbemycin oxime and spinosad in it (group 2), another given a tablet with ivermectin and pyrantel (group 3), the third medicated group was given a selemectin spot-on (group 4) and the final medicated group was given a spot-on containing moxidectin and imidacloprid (group 5).

Again, each dog was inoculated with live heartworms and if they were in a medicated group, they received their treatments and then again on a monthly basis until the end of the study. At the four-month mark, as with the other study, all of the control dogs still had considerable heartworm burdens.

Of all four of the monthly treatment groups, only the moxidectin plus imidacloprid group had total elimination of heartworms in each patient. The other three monthly preventative groups had either whole adult worms or fragments of worms still circulating in the patient at the end of the study despite receiving monthly “preventatives”.

In fact, this study demonstrated an even lower efficacy than the first study mentioned! Group three, who was given the ivermectin-containing product, showed that 70% of the patients weren’t adequately protected, while the milbemycin group (group 2) showed that just over half of the animals in the group were actually protected fully against heartworm.


Overall, it is incredibly important that you, the pet owner, are aware of the active ingredients being used in any of the preventatives that you give your dog. It is especially important in this case to consider the efficacy of these products, as more and more independent studies are being released demonstrating less than ideal results when it comes to monthly heartworm treatments.
The Proheart yearly heartworm injection contains the active ingredient moxidectin in a slow release form, which therefore covers the recipient for an entire year rather than just one month. Even better still, the Proheart injection has what is called a “reachback” period, meaning that even if you are a couple of months late for your appointment, your dog will still be covered against heartworm.
So, not only does the annual injection mean fewer vet visits and fewer tablets forced down throats, but it also means greater protection for your dog against heartworm. In fact, the Proheart annual injection is the only heartworm preventative product available with a proven 100% efficacy rate in terms of both prevention and treatment.

If you want to discuss change heartworm products with a veterinarian, please contact us to arrange a free heartworm consultation.




Heartworm Surveillance Project – available from www.vetsaustralia.com.au